Function dram_streams 

pub fn dram_streams(
    mp: &Metapipeline,
    buffer_words: &[usize],
    ag_stages: &[usize],
    t: &PlasticineTarget,
) -> Result<(usize, usize), Unplaceable>

Perimeter DRAM ports one instance of mp needs, (into the mesh, out of the mesh): every buffer edge written by a perimeter-hosted stage is one inbound stream, every edge read by one is one outbound stream (each PMU of a spanning buffer is its own stream).